Automated material handling and sortation systems are known for receiving, transporting and discharging goods among various stations in large scale sortation operations such, for example, as warehousing, distribution, postal sortation and handling of mail and packages, and airport baggage handling. Whatever the operation, goods typically originate from one location within the facility and must be sorted and transported to several different locations for further handling, or originate from several locations within a facility and must be transported to a single location such as a shipping dock. The manner in which the various goods are stored and selectively distributed among various stations in a facility of course depends on the nature of the operation.
One known sortation and delivery method involves using powered belt or roller conveyers to transport individual items or sorted loads of items to various destinations within a facility. When goods from multiple sources must be delivered to a single station, associated take away conveyors must be merged onto a main conveyor or discharge point. This requires careful coordination of each item as it arrives to prevent jams or damage. Each merge point on such a conveyor system accordingly requires a complex system of sensors, start/stop controls, actuators, power supply lines, etc. Similarly, when items must be delivered to multiple destinations or stations in a facility, a main conveyor must be provided with diverter apparatus to direct individual items or batches of items to either continue or be diverted at various points. Each diverter apparatus requires an additional closed system including sensors, actuators, control mechanisms, wiring power supply to accomplish the diverting operation and track and identify the items being diverted.
The disadvantages of conveyor-type systems have led to the development of tracked systems in which a closed loop track carries cars propelled by a continuous chain drive. The cars are equipped with open trays which can be loaded from belts or chutes, and subsequently tilted to unload their carloads into bins which are located around the track. Such systems are designed for long term installations which sort and transport large volumes of goods. Although these closed loop track systems are an improvement over conveyor-type systems, the complexity of their track, drive and tilting mechanisms makes it a major undertaking to set them up or rearrange their sortation layout. Further, they must be totally shut down for nearly all maintenance tasks.
An improvement over continuous belt or chain drive closed loop track systems is disclosed in U.S. Pat. No. 5,018,928 issued to Hartlepp and assigned to the assignee of the present invention. Hartlepp discloses a train-type automated track sortation system, including a number of trolley cars and a locomotive or tug for pulling the cars around the track between various discharge and induction stations. The cars on the train are loaded with goods to be sorted or delivered and given destination addresses for the goods at an induction or loading station. Once a train has left the loading station, functions such as keeping track of position, regulating speed and dumping cars are controlled by a microprocessor placed aboard each locomotive.
Although train-type automated track sortation systems of the type disclosed by Hartlepp represent an improvement over conveyor-type and belt or chain-driven track systems, there is need for an improved tug or locomotive to pull the train. Prior art tugs or locomotives typically include front and rear trucks supporting the locomotive and a traction or drive unit incorporated in one of the trucks. Whereas this arrangement is generally satisfactory, it in effect integrates two distinct functions-the provision of a pulling force for the train and the provision of a support for the main body of the locomotive-in a single unit with the result that neither function can be optimized.